With reference to FIG. 1, a ducted fan gas turbine engine generally indicated at 10 comprises, in axial flow series, an air intake 1, a propulsive fan 2, an intermediate pressure compressor 3, a high pressure compressor 4, combustion equipment 5, a high pressure turbine 6, an intermediate pressure turbine 7, a low pressure turbine 8 and an exhaust nozzle 9.
Air entering the air intake 1 is accelerated by the fan 2 to produce two air flows, a first air flow into the intermediate pressure compressor 3 and a second air flow that passes over the outer surface of the engine casing 12 and which provides propulsive thrust. The intermediate pressure compressor 3 compresses the air flow directed into it before delivering the air to the high pressure compressor 4 where further compression takes place.
Compressed air exhausted from the high pressure compressor 4 is directed into the combustion equipment 5, where it is mixed with fuel that is injected from a fuel injector 14 and the mixture combusted. The resultant hot combustion products expand through and thereby drive the high 6, intermediate 7 and low pressure 8 turbines before being exhausted through the nozzle 9 to provide additional propulsive thrust. The high, intermediate and low pressure turbines respectively drive the high and intermediate pressure compressors and the fan by suitable interconnecting shafts.
Compressor and turbine aerofoils either on rotating blades or static or variable vanes can be damaged in use which require their repair. Where repair is required for the aerofoil the repair process can involve removal of a portion of the aerofoil down to a blade stock and then using an additive manufacture process to provide a replacement portion by depositing layer upon layer of material to the blade stock.
Additive manufacture processes are known in the art and fall into a number of broad methods. In the first method, commonly known as powder bed processing, a layer of powder material is supplied over the surface of the blade stock and a laser is traversed over the powder to partially or fully melt the powder at selected positions which joins the powder to the blade stock or an underlying powder layer. The blade is indexed away from the surface and a new layer of powder supplied over the previous layer and the laser traversed over the surface to repeat the melting process. The steps of indexing, powder laying and melting are repeated till the blade is complete.
In a further method of additive manufacture commonly known as direct laser deposition a laser is traversed over the surface of the blade stock with sufficient energy to form a pool of molten material. Into the melt pool a material is supplied either in powder or wire form which is melted by the pool. As the laser traverses away from the melt pool the material cools and solidifies to form a deposit with a height. Repeated passes of the laser over the deposit and further deposition increases the height of the deposit till the blade is complete.
FIG. 7 depicts is a schematic drawing of a DLD apparatus in which a laser generator 100 directs a laser beam 102 towards a structure. A substrate 110 is mounted to a table 112, moveable relative to a laser 100 and a powder delivery nozzle 116. The method of forming a structure 118 comprises directing a beam from the laser 100 onto the substrate 110 or later the forming structure 118, to create a pool of molten metal 122 into which a powder 124 is directed as a jet. Once sufficient powder has been deposited a relatively thin layer of metal remains. The substrate 110 and forming structure 118 are translated so that the structure is formed in layer-wise manner. The process allows a near net material direct manufacture of structures. By controlling the amount of powder and the location of the base material simple and complex structures may be formed. For gas turbine engine blades and the like, one advantage of this process is that complex aerofoil shapes can be manufactured directly from a computer aided design model without the need for traditional process steps. It is an essential part of this process that the laser, delivery of powder and location of the deposit are computer 132 controlled.
The build or re-build of an aerofoil edge and particularly a full chord build requires careful control of the laser position as each layer of material is deposited. Each aerofoil, however, can have slight differences in their build from wear or from its original manufacture which means that it may not be possible to use a “nominal” CAD model of the aerofoil to which the build is applied. If the laser position is not accurately controlled then the geometry of the deposition may make it impossible to re-profile the deposit to acceptable aerofoil dimensions. Such a failure would create performance and quality concerns to the compressor or turbine into which the aerofoil is supplied.
It is an object of the invention to seek to provide an improved method of determining and/or generating adaptive chordal tool paths for additive manufacture to aerofoils.